Optical shape sensing (OSS) is an optical measurement technique for determining the position and shape of a structure in a three dimensional space. In particular, optical shape sensing can be applied for minimally invasive procedures in health care, wherein it is advantageous to monitor the three dimensional shape of an elongated medical device, e.g. endoscopes, catheters or guidewires, with minimal use of harmful radiation.
To this end, an optical shape sensing technology is developed based on back scatter reflectometry. The light of a light source is split into a reference signal and a device signal, wherein both signals are guided in fibers through an interferometer. The device signal is guided first to undergo scatterings within the fiber portion that is inserted into a medical device and subsequently to interfere with the reference signal due to their different optical paths. Based on the interference signal, information regarding mechanical strains due to shape deformations of the device can be retrieved.
Shape deformations not only give rise to length variations of the medical device but also to changes in the refractive index. Moreover, such changes are usually not isotropic, but depend on the polarization of the impinging light. For instance, the impinging light polarized parallel to the direction of curvature has a different refraction index from the impinging light polarized perpendicularly to the direction of curvature.
This phenomenon is known as birefringence, which has two effects for the optical shape sensing. First of all, it changes the state of polarization of the scattered, in particular reflected signal dependent on the position of the scattering point within the device. In particular, the total optical path from the laser source to the scattering point and subsequently towards the detector is dependent on the actual position of the scattering point. For a given polarization of the input light the detected light from some of the scattering points may have a state of polarization that is orthogonal to the state of polarization of the reference signal. This leads to polarization fading, due to which no information can be retrieved for such scattering points.
In addition, a change in the refraction index cannot be disentangled or decoupled from a mechanical length change and will influence the phase of the interference signal. Consequently, the result of the strength measurement as well as the optical shaped sensing will be no longer reliable. Moreover, due to the non-isotropic nature of birefringence, the interference signal will depend on the exact state of polarization set for the laser source and possibly also on the exact state of polarization of the detector arrangement.
U.S. Pat. No. 8,773,650 B2 discloses an accurate measurement method and an apparatus for shape sensing with a multi-core fiber. A change in optical length is detected in one of the cores in a multi-core fiber up to a point on the multi-core fiber. A location and/or pointing direction are/is determined at the point on the multi-core fiber based on the detected changes in optical length. The accuracy of the determination is better than 0.5% of the optical length of the multi-core fiber up to the point on the multi-core fiber. In an example embodiment, the determining includes determining a shape of at least a portion of the multi-core fiber based on the detected changes in optical length.
EP 2 720 388 A1 discloses an optical frequency domain reflectometry system comprising a first coupling point arranged for a splitting radiation into two parts, so that radiation may be emitted into a reference path and a measurement path. The system further comprises an optical detection unit capable of obtaining a signal from the combined optical radiation from the reference path and the measurement path via a second coupling point. The measurement path comprises a polarization dependent optical path length shifter, which may create a first polarization and a second polarization in the measurement path, where the optical path length is different for the first and second polarizations in the measurement path.
WO 2010/073002 A1 discloses a distributed optical fiber sensor which uses a sensor fiber having a low or zero intrinsic birefringence that is responsive to an environmental parameter such as pressure. Probe light pulses having a diversity of launched polarization states are used to reduce signal fading and polarization dependent loss in the retardation speed frequency signals which are sensed and then analyzed to determine the environmental parameter as a profile along the sensor fiber.